1. Field of the Invention
The present invention relates to methods and related apparatus for retrieving a pump from a wellbore and, more particularly, to retrieving a pump while leaving the pump's drive mechanism within the wellbore.
2. Description of Related Art
Subterranean fluids, such as oil, gas and water, are often pumped or "lifted" from wellbore by the operation of downhole pumps, such as by electric submergible pumping systems. These pumping systems typically use an elongated electric motor installed within the wellbore to rotate a multistage centrifugal pump. While centrifugal pumps are widely used for the recovery of subterranean fluids, such centrifugal pumps have difficulty in lifting viscous fluids, such as from Southern California, and fluids with relatively high concentrations of sand and other abrasive materials, such as from the tar sands area of Alberta, Canada. Thus, there is a need for a downhole pump that can lift such fluids.
A solution to the problem of recovering viscous fluids and fluids with relatively high concentrations of sand, consists of using a Moineau pump or a progressive cavity pump. Conventional installations of progressive cavity pumps place the drive means at the earth's surface. A rod string which is used as a drive shaft rotates inside the production tubing. In wells that are deviated and/or produce abrasives, the rotating rod string causes production tubing wear. The frequent replacement of production tubing is very expensive and can cause a well to be uneconomic.
A problem encountered with progressive cavity pumps is that the seal formed between the rotor and stator wears away, reducing the pump's efficiency until it eventually stops pumping fluid, thus the pump needs to be retrieved from the wellbore periodically. Since the pump is rigidly connected to the downhole drive mechanism, when the pump is retrieved the entire downhole drive mechanism is also retrieved, which is a time consuming and a relatively expensive operation that requires a workover rig. The downhole drive mechanisms have operational lives many times longer than the progressive cavity pump, so there is a need for a method and apparatus for retrieving the pump alone and while keeping the downhole drive mechanism within the wellbore. With such a method the size of the pulling unit can be reduced, and thereby save time and money.